Armin could be the storyteller. Sometimes characters take on this role to provide a unique viewpoint or to drive the narrative in a particular direction. But we need to know more about the story to confirm it for sure.
Armin is still alive in the manga. The story developments don't result in his demise. His presence remains significant and continues to shape the narrative.
One common theme is friendship. They both come from different backgrounds and have had different experiences, so exploring how they form a friendship is popular. For example, in many fanfics, they start off as just acquaintances and gradually become close friends through shared experiences.